1. Field of Invention
The present invention relates to linear actuators utilizing active materials, more specifically to shape memory alloy actuators for use in controlling the displacement of mechanical systems.
2. Prior Art
Modern, non-electromechanical actuators, such as Shape Memory Alloys (SMA) and Electroactive Polymer (EAP) Actuators, convert the source energy to mechanical energy based on inherent material properties that exhibit actuator functionality. Although these actuator materials have lots of possibilities, numerous difficulties persist in using them reliably and suitably as robot actuators. Shape Memory Alloy (SMA) actuators, for example produce the one of the highest stresses among all actuators that has ever been developed. Its maximum actuation stress of over 200 MPa is 570 times larger than the human muscle, and 25 times larger than the latest electroactive polymer actuators. Its energy density of over 100 Joule/cm3 is 100 times larger than that of piezoelectrics. These high stress and energy density characteristics allow SMA actuators to be effectively used in various applications where space and weight constraints are critical design requirements. These include medical devices, robots, and smart structures.
Despite the tremendous actuation stress and energy density, SMA has highly complex nonlinear dynamics that limit applicability and utility to rather simple tasks. Controlling the displacement of SMA is not easy. In the past few decades a number of research groups have modeled the SMA thermo-mechanical behavior in order to accurately control the SMA. These include finite element methods based on the Galerkin method, Preisach approaches, and models based on thermodynamic principles and constitutive equations. Researchers also have attempted to compensate for these thermo-mechanical nonlinearities utilizing nonlinear control approaches: neural networks and a sliding mode based robust controller, neural fuzzy, dissipativity, and variable structure control. Despite these valuable research efforts, control of SMA is still difficult. Fundamental control performance, e.g. speed of response and disturbance rejection, is still limited even when complex models and sophisticated controls are used.
The fundamental difficulties of SMA control include:                The phase transition diagram shows a prominent hysteresis with transitional regions that are both steep and nonlinear.        The phase transition temperatures shift as the stress and environmental conditions change.        The process is distributed and thereby phase transition is not uniform along the SMA wire.        
As mentioned previously, these difficulties are managed with various nonlinear control methods with detailed models of the SMA phase transition. But the models are not reliable and require knowledge of accurate parameter values.
Prior art U.S. Pat. No. 5,763,979 controls multiple independent SMAs by implementing a matrix actuation system. The SMAs are put into a matrix, and based on a row and column input, a computer pulses current to a specific segment for actuation. They emphasize that this reduces the amount of wiring to actuate a large number of SMA elements. They independently actuate specific segments but those segments are not put in series to create a linear actuator. Their claim is based around reducing the number of wires in an application that uses many shape memory alloys but not any application or purpose.
Prior art U.S. Pat. No. 6,574,958 is a displacement amplifying binary actuator. Their invention puts many SMA geometrically in parallel and electrically in series. Only two discrete displacements can be created by this invention, like most of other SMA related inventions.
Prior art U.S. Pat. No. 6,133,547 regards using numerous thermoelectric modules to heat and cool a sheet of SMA. By turning on thermoelectric modules, the local shape changes, and thus the overall shape of the sheet is altered. This invention does not create a linear displacement.
Prior art U.S. Pat. No. 4,553,393 puts SMA elements mechanically in parallel for the effect of controlling the force that is applied to the linear displacement output. In addition, this invention utilizes Peltier elements for decreasing the time it takes the SMA to cool down. This invention utilizes multiple SMA elements to control force, not displacement.
Shape memory alloy actuators found in the prior arts are generally for switching, or they are to be controlled as a whole. There is a need to use SMA actuator as a linear actuator that can control the displacement easily. Because the prior art devices rely on the nonlinear relationship between the temperature and the length of the SMA wire, the accuracy of the control will be degraded when there is a load change during operation. Also when controlling the displacement using resistance or displacement of the actuator, the temperature of the wire can increase to very high temperature, which will degrade the actuator and make the actuator unusable. Also, in order to maintain a certain state, the actuator has to be provided with a constant amount of energy, or the state will fluctuate due to the hysteresis inherent in the shape memory alloy. This is why most of the SMA actuator applications are limited to switching, rather than creating a continuous displacement.